Brushes in motor worn low, check length, compare to a new set, low brushes do not contact the com and cause excessive arcing at the com,I bet you when you take out the brushes the end of one will be nice and polished the other will be dull .the dull one is not contacting the com hence faulty (they may both be dull.)New set cures problem for 12 months happy wife,
wirecutterman
December 2007